1. Field of the Invention
The present invention relates to a method of patterning a transparent conducting film and particularly to a method of patterning a transparent conducting film formed on a substrate including silicon.
For a light detecting part such as an imaging device, an amorphous silicon (a-Si) film suitable for receiving the visible light, an amorphous silicon carbide (a-SiC) film or a multilayered film thereof is of them are used and as a transparent conducting film, an indium (In).tin (Sn).oxide film (ITO film), for example. Therefore, processing of such a film is a very important fabrication process in the imaging device fabrication process.
2. Description of the Related Art
FIGS. 1(a).about.(e) are schematic sectional views for explaining a part of the conventional fabrication process of an imaging device.
FIG. 1(a) shows a sample immediately before formation of a transparent conducting film, where a multilayered film consisting of a-SiC film/a-Si film/a-SiC film is formed as the light receiving part. The structures shown in FIGS. 1(a).about.(e) are called the "sample" in the respective steps.)
In FIG. 1(a), the reference numeral 1 denotes a silicon substrate; 2, a silicon oxide film (SiO.sub.2 film) for device separation; 3, an interlayer insulation film; 4, a transistor for amplification; 5, Al wiring; 6, a PSG (phosphosilicate glass) film for flattening; 7, a light receiving part consisting of an a-SiC film 7d/a-Si film 7c/a-SiC film 7b. In this FIGURE, a collector electrode for the charges generated through photoelectric conversion, a charge storage diode and a CCD (Charge Coupled Device) for transferring charges have the well known structure and these are not illustrated.
As shown in FIG. 1(b), a sample of FIG. 1(a) is put into a sputtering apparatus, substrate temperature is raised (up to about 150.degree. C.) and thereafter a first transparent conducting film 8 consisting of ITO is formed by the sputtering on the a-SiC film 7d. Here the substrate is heated to improve adhesive strength between the substrate and the transparent conducting film and also to improve light receiving characteristics.
Next, as shown in FIG. 1(c), a resist pattern 9 is formed on the first transparent conducting film 8 and this first transparent conducting film 8 is then removed by etching using the aqueous solution of ferric chloride (FeCl.sub.3) with the resist pattern 9 used as the mask. Such wet etching is carried out here because since the vapor pressure of indium (In) and tin (Sn) is low, and consequently an etching rate is low for the dry etching of ITO.
As shown in FIG. 1(d), after the a-SiC film 7d/a-Si film 7c/a-SiC film 7b is etched by nitrogen trifluoride (NF.sub.3), the PSG film 6 on the Al wiring 5 is selectively etched and thereby an aperture 10 is formed. Thereafter, this sample is put into the sputtering apparatus and a second transparent conducting film 11 is formed on the sample surface under the same conditions as as existed during formation of the first transparent conducting film 8. Thereby, the Al wiring 5 is connected to the second transparent conducting film 11 through the aperture 10.
Finally, as shown in FIG. 1(e), a resist pattern 12 is formed on the second transparent conducting film 11 and the second transparent conducting film 11 is etched by the FeCl.sub.3 aqueous solution with the resist pattern used as the mask. As explained above, a leadout electrode 11c is formed, completing an imaging device.
In above fabrication process, since the sample is kept at about 150.degree. C. during formation of the first transparent conducting film 8 of the light receiving part, an interfacial reacted layer 8a is generated by reaction ITO and Si at the interface between the first transparent conducting film 8 and a-SiC film 7d, as shown in FIG. 2(a). This interfacial reacted layer 8a by reaction does not easily dissolve into the aqueous solution of FeCl.sub.3. Therefore, when the etching time is set longer in order to perfectly remove the interfacial reacted layer 8a shown in FIG. 2(a), the lateral etching of the first transparent conducting film 8b, where ITO does not react with Si, proceeds as shown in FIG. 2(b), and a part A, where only the interfacial reacted layer 8a remains on the a-SiC film 7d, is generated. When this condition is observed from the direction where the light is incident, the periphery of the first transparent conducting film is partly protruded and recessed. Therefore, the protruded and recessed regions are also generated in the periphery of the a-SiC film 7d/a-Si film 7c/a-SiC film 7b by the successive etching. As a result, a leak current of the imaging device fabricated increases.
In order to avoid such phenomenon, the ITO film is formed under a lower temperature, then the ITO film may be peeled easily and light transmissivity becomes small so that it can no longer be put into the practical use.
As shown in FIG. 1(d), the phenomena conducted on the first transparent conducting film 8 are also generated even after formation of the second transparent conducting film 11 made of ITO film. Namely, as shown in FIG. 3(a), a side product layer 11a is generated by reaction of ITO and Si included in the PSG film 6 at the interface between the second transparent conducting film 11 and PSG film 6. Therefore, if it is attempted to perfectly remove the interfacial reacted layer 11a at the time of etching the second transparent conducting film 11 as shown in FIG. 1(e), only the ITO film 11b is etched in the lateral direction shown in FIG. 3(b), and consequently the etchant sometimes reaches the Al wiring 5. In this case, the Al wiring 5 may be etched because the etchant includes chlorine. As a result, a problem occurs in that quality of imaging device is deteriorated occurs.
As explained above, a method of etching the transparent conducting film, such as ITO, to be used for the imaging device, which provides an adequate etching rate and giving which produces no adverse effect on the device characteristics, has been not found yet.